The present invention relates to locating a region within a three-dimensional volume. For two-dimensional medical imaging, the distance between two points is easily determined. The endpoints are readily identified within the scan plane of the image, such as by user selection, and the scan parameters are used to identify a distance between the two points. Similarly, the user can assist in determining a region of interest by selecting one or more points along the desired border. Since the scan plane associated with the image is well defined, identification of points or other regions within the scan plane may be made with minimal error in reference to the patient location. However, two-dimensional representations of a three-dimensional (3D) volume introduce ambiguity. Since the 3D volume is viewed on a two-dimensional device, selection of a position on the image corresponds to a plurality of different locations. For a given pixel, the pixel locations on the viewing display represents a line in a viewing direction used for rendering the representation of the three-dimensional volume.